A variety of sensors can be utilized to detect conditions, such as pressure and temperature. The ability to detect pressure and/or temperature is an advantage to any device exposed to variable pressure conditions, which can be severely affected by these conditions. An example of such a device is a catheter, which of course, can experience variations in both temperature and pressure. Many different techniques have been proposed for sensing the pressure and/or temperature in catheters, and for delivering this information to an operator so that he or she is aware of pressure and temperature conditions associated with a catheter and any fluid, such as blood flowing therein.
One type of sensor that has found wide use in pressure and temperature sensing applications is the Surface Acoustic Wave (SAW) sensor, which can be composed of a sense element on a base and pressure transducer sensor diaphragm that is part of the cover. For a SAW sensor to function properly, the sensor diaphragm should generally be located in intimate contact with the sense element at all pressure levels and temperatures.
One of the problems with current SAW sensor designs, particularly those designs adapted to delicate pressure and temperature sensing applications, is the inability of conventional SAW sensing systems to meet the demand in low pressure applications. (e.g., 0 to 500 mmHg), while doing so in an efficient and low cost manner. Such systems are inherently expensive, awkward, and often are not reliable in accurately sensing air pressure and temperature. There is a continuing need to lower the cost of SAW sensor designs utilized in pressure and/or temperature sensing applications, particularly wireless pressure sensors.
To lower the cost and raise efficiency, few components, less expensive materials and fewer manufacturing-processing steps are necessary. In order to achieve these goals, it is believed that a disposable SAW pressure sensor made of polymer substrate should be implemented, along with low cost processing steps. To date, such components have not been adequately achieved.
One area where the ability to detect pressure and/or temperature is critically important is in the field of medical applications. Pressure within a conduit, for example, such as a catheter, can be measured utilizing a number of techniques. Perhaps the most common device for such measurement is a mechanical gauge, which can be coupled through one wall of the conduit to the fluid pressure within the conduit. Inside the gauge, a needle is deflected over a scale in proportion to the pressure within the conduit. In some instances, the standard pressure gauge may be replaced with a transducer, which converts pressure into an electrical signal, which is then monitored. One important medical application for a pressure sensor involves detecting a patient's blood pressure, and/or intracranial pressure.
One typical method of monitoring blood pressure is to measure the fluid pressure within an intravenous tube, which is hydraulically coupled to the patient's vein. A catheter is inserted into the patient's vein and a plastic tube or conduit coupled to the catheter. A saline solution can be drip-fed through the plastic tubing or conduit to maintain a pressure balance against the pressure within the patient's vein. The saline fluid acts as a hydraulic fluid to cause the pressure within the plastic tubing to correspond to the pressure within the patient's vein. Hence, by measuring the fluid pressure within the tubing, the patient's blood pressure will be known.
Various conventional SAW sensing devices are capable of measuring blood pressure. Such devices typically are configured from ceramic materials (like PZT), quartz-type piezoelectric materials or lithium niobate. Such devices are disadvantageous for medical applications, because the above-referenced materials utilized by such devices are inherently self-resonant, having extremely low piezoelectric coupling coefficient, expensive and difficult for micro-machining, and consequently, grossly reduce the possibility of making a low cost pressure sensor for medical applications.
Conventional quartz-based SAW pressure sensors are also expensive to implement in medical applications, rendering their widespread use limited. Micro-machining in quartz is nothing close to that of silicon. It is therefore believed that a solution to such problems involves a disposable low cost sensor packaging system, particularly one that is suited to medical applications.